SASS-styles with ui mixins doesn't slicing properly for legacy IE during sencha app build. I created new application "Bugreport" and changed default sass namespace to undefined, because i want to style components with both namespaces, Ext and Bugreport. Then, I write style for ExtJS panel using ui mixin. It works and looks right in modern browsers, but not in old IE. Older IE are emulating in IE11 with Developer Tools IE5-8 modes.

Have you added your custom UI to the manifest for sass/example/theme.html? Unless you do, Sencha Cmd won't know which UIs to generate images for. There's a guide on how to add your UI to the manifest in the "Slicing Images for CSS3 effects in IE" section of the theming guide.